Amplifier using gas-filled tube



Aug. 18, 1942. G, R, sTlBn-z 2,293,570

AMPLIFIER UsgNG GAS-FILLED TUBE Filed Nov. 7, 1940 F/G./` Q6 HG2 HomeHung VVE/WDA7 G. R. STIB/TZ Patented Aug. 18, 1942 AMPLIFIER USINGGAS-FILLED TUBE George R. Stibitz, Boonton, N. J., assignor to BellTelephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application November 7, 1940,Serial No. 364,633

2 Claims.

. tinguished by the alternating or interrupted high frequency voltageapplied across the discharge electrodes, and the phase in the highfrequency cycle at which the discharge is extinguished is ordinarilyconstant, the exact value depending on the tube design, including thekind of gas used. By controlling the phase in the cycle at which thedischarge begins, the fraction of the cycle during which current flowsis controlled and this in tum determines the useful load current.

.Filtering means is used to keep the high frequency current out of theload circuit.

It has been suggested that the high frequency voltage wave be shaped toreduce distortion. This involves use of shaping devices that are likelyto be cumbersome and costly.

In accordance with the present invention distortion is reduced byfeeding back a portion of the output wave yor voltage to advance orretard the phase in the cycle at which breakdown occurs, and by makingthe effect of the fed back wave or voltage opposite to that of theoriginal input wave or voltage whereby the over-all amplification isreduced as compared to the case of no feedback. Since the gainobtainable with a gas-filled tube is very large, large reduction in gaincan be produced by the feedback action and this means that largeincrease in linearity is effected. The circuit connections for producingthe feedback action are simple in comparison with wavel shaping circuitsthat have been suggested heretofore.

'I'he nature of the invention and its various objects and featureswill'appear more fully from the following detailed description ofillustrative embodiments shown in the accompanying drawing.

In the drawing:

Fig. 1 is a schematic circuit diagram of one form of circuit usingnegative feedback in acordance with this invention;

Figs. 2, 3 and 4 are similar diagrams of alternative circuits; and

Figs. 5 and 6 are similar diagrams showing the use of two tubesdifferentially coupled to. a common load.

In Figs. l, 2, 5 and 6 cold cathode gas-filled tubes are shown, althoughin all of these circuits hot-cathode tubes could be used if desired.

In Fig. 1, the gas-filled tube I has main discharge gap 2, 3 and astarting gap 2, 4. Electrodes 2 and .4 maybe alike and are locatedrelatively close to each other. Electrode 3 is physically distinct andis always the anode. In Fig. 1 the input is connected across 2 and 4 sothat the discharge is initiated across the gap 2, 4 supplying ions toset ofi the main gap 2, 3.

The signal applied across terminals 5 may consist of varying directcurrent, or impulses, speech or other suitable type. 'I'he source ofalternating current 6 is of higher frequency than the signal and may beas high as desired within the limitation that the time required torestore the tube to unionized condition must be small compared with ahalf period of the high frequency wave. This wave is applied throughtransformers I and 8 to both the input circuit and the output circuit.

The input circuit is connected to the cathode by Way of series resistor9 and the slider on resistor I0 across which the load terminals II `areconnected, shunted by by-pass condenser I2 for the high frequency wavefrom 6. Similarly the input terminals are shunted by by-pass condenserI3 for the high frequency wave.

The resistance IIJ is in part common to the output circuit and inputcircuit and provides negative feedback as will be described.

In the absence of any signal voltage at terminals 5, 5 there will berectified high frequency current across the resistance I0 and across theload terminals but the condenser I2 will by-pass the high frequencycomponent. More complete filtering may be used as desired at this point.The high frequency Wave drives electrodes 3 and 4 both positive at thesame time and both negative a half cycle later. In the positive halfcycle current flows for a certain fraction of the time depending uponadjustments. In the negative half cycle the discharge is extinguished inboth the starting gap and the main gap.

A signal which makes the starter electrode 4 more positive advances thetime in the high frequency cycle at which the discharge begins. Once thetube has broken down the starting electrode loses control and the tubeis restored at a given point in the high frequency cycle which for agiven tube and circuit remains substantially constant. A signal whichmakes the starter electrode 4 more negative delays the breakdown until alater time in the high frequency cycle. Thus, during a positive swing ofthe signal voltage a succession of current impulses is produced in theoutput circuit of greater than normal duration and during a negativeswing of the signal voltage the output high frequency impulses areshorter than normal. The cumulative effect is a charge on condenser I2,hence across the load,` which varies in accordance with the signalvariations.

Since only a relatively small input voltage is required to control alarge energy ilow in the output circuit, a large factor of amplificationis possible with this type of circuit. With a high frequency wave ofsinusoidal shape or any shape in which the phase angle at whichbreakdown occurs is' not strictly proportional to the peak inputvoltage, a considerable distortion is produced in the amplified outputsignal or indication. For example, assuming a sine wave form for thehigh frequency voltage, the effective amplification increases markedlywith input peak signal voltage and may be many times :as great for a.high amplitude signal as for a low amplitude signal.

In accordance with the invention this distortion is reduced by applyingto the starter electrode some of the output signal voltage in oppositephase to the incoming signal, and in Fig.. 1 this feedback voltage isobtained from the resistance I8, the amount depending upon the positionof the slider along the resistance. This fed back voltage represents theamplified signal distorted to some extent. 'If the distortion at anyportion of the signal swing is of such nature that the current isflowing for too large a part of the high frequency cycle, the fed backvoltage tends to delay the breakdown point in the high frequency cycleso that the discharge impulse is shortened, and if the impulse isdisproportionately short the fed back voltage advances the breakdownphase to lengthen the impulse. This action comes about automaticallyfrom the fact that the fed back voltage is substantially opposite inphase to the incoming signal voltage. Preferably the feedback voltageratio is relatively large in order to secure a correspondingly greatreduction in gain and improvement in distortion. The use of negativefeedback for reduction of distortion is broadly claimed in H. S. BlackPatent 2,102,671, patented December 21,

The circuit of Fig. 2 is similar to that of Fig. 1 -except that in Fig.2 separate impedances are used for the feedback impedance I5 and for theload impedance I1. These are shunted respectively by by-pass condensersI6 and I8 for the high frequency. The operation is otherwise the same.

In Fig. 3, since a. hot cathode tube 20 is used, it is unnecessary toextinguish the discharge between the control electrode and cathode, andtransformer 1 can thus be dispensed with. The circuit is of the platefeedback type, meaning these impulses are shortened and during thepositive signal swing they are lengthened. The K negative feedbackeffects a corrective phase displacement in such direction as to reducethe `output and at the same time and in corresponddensers shown arelby-pass condensers for the that the feedback impedance I9 is next to theplate. Bias battery 22 is used to apply a negative bias to the controlelectrode relative to the anode. Condenser 2| by-passes the highfrequency and the load impedance is shown at 23 comprising a resistorand shunting condenser. In the absence of a signal voltage dischargeimpulses of slightly less than half a cycle duration are produced.vDuring the negative signal swing high frequency wave from source 6.Condenser 26 also serves as a grid bias capacity in conjunction withresistance 24.

In some cases, such as in measuring circuits, it is desirable to makethe circuit immune to fluctuations in the peak voltage of the highfrequency source 6. This is done in Figs. 5 and 6 in which two tubes 21and 28 are used to cancel out the effects of such voltage changes.

In the absence of a signal or voltage to be indicated or measuredapplied to the "in terminals, the adjustments are such that both tubesbreak down at the same instant and produce the same potential at bothterminals of the load or meter so that for a constant or varying valueof peak voltage of the source 6 thereis no difference of potentialacross the load terminals. The sliders on resistances 29 and 30 may beadjusted to correspond to this balanced condition.

If, now, in Fig. 5 a signal or other voltage is applied to the input,the phase in the high frequency cycle at which tube 21 breaks down isvaried while it remains fixed in the case of tube 28. The current in thetwo portions of resistance 29 becomes unbalanced and this causes currentto iiow into the load. The voltage in either half of resistor 29 alsoproduces feedback to the corresponding grid through the correspondingcondenser 3I or 32, and the size of the fed back voltage is determinedby the ratio of the two resistances 33 and 34 for tube 28 andresistances 35 and 36 (including the resistance across the inputterminals) for tube 21. Condensers 3I and 32 also provide a path forapplying negative restoring voltages to the starter electrodes.

The operation of Fig. 6 is similar to that of Fig. 5, the maindifference being that the high frequency voltage is in Fig. 6 applied tothe starter electrodes through condensers 38, 39, 40 instead of througha transformer. The high frequency voltage applied to the grids is variedby the slider on resistor 30. A filter 4I is shown in the input circuitfor purifying the wave to be measured.

In the absence of an input voltage both tubes break down as abovedescribed, producing no meter currentI since equal currents flow throughequal resistances 45 and 46. A voltage across the input terminals variesthe starting time of the impulses in tube 21 but not in tube 28, andthis causes an equalizing current to iiow through the meter 50. On thenegative high frequency cycle, restoring voltage is applied throughcondensers 39 and 40 for restoring the main gaps and through condensers3| and 32 for restoring the starter gaps. Negative feedback is producedby applying voltage from across each resistor 45, 46 to thecorresponding starter electrode, this voltage being momentarily storedin the corresponding condenser 3 I or 32. In the case of tube 21 thevoltage is applied through the resistors 42 and 43 and the inputterminals and thence through resistor 35. In the case of tube 28 thevoltage is applied through series resistors 3l and 33.

The circuits that have been disclosed are to be taken as illustrativerather than limiting. The scope of the invention is dened in the claims.

What is claimed is:

1. In combination, a circuit comprising a pair of gas-filled tubes, aload differentially coupled to l the discharge space electrodes of saidtubes, a source of intermittent voltage for supplying the dischargespaces of vsaid tubes in parallel, an input circuit for voltages to beamplied coupled to the control elements of one only of said tubes, andmeans for feeding back from the output sides of said tubes to thecontrol electrodes of said tubes a portion of the output voltage in suchphase as to tend to oppose the initiation of current iiow in saiddevices and `in sumcient amount to improve the linearity of amplication'of 'the circuit. v

2. In an amplier circuit, a pair of gas-filled tubes each havingdischarge electrodes and a starter electrode, means to apply a voltageto be amplified to the starter electrode of only one of said tubes, asource of alternating voltage, connected to the discharge electrodes ofsaid tubes, of a frequency high in comparison to the frequency of saidvoltage to be amplified, a load circuit diierentially coupled to thedischarge electrodes of both tubes, means to apply a portion of saidalternating voltage to said starter electrodes in like phase, and meansto feed back a portion of the load voltage to the starter electrode ofsaid one tube in phase opposition to the voltage that is to beamplified.

. lGEORGE R. STIBITZ.

